From around mid-2013 MB started fitting cars with the 'Mercedes me' feature, which uses a built-in KOM Module to allow the car to connect to the Internet (via a built-in Vodafone data-only simcard).
'Mercedes me' comes with a range of services that require paid renewal.
For older cars, however, MB reased the confusingly-named 'Mercedes me Adapter', which is a simple OBDII dongle that can connect to your phone via Bluetooth.
The 'Mercedes me Adapter' is currently handed-out for free, and has no services related to it so there are no running costs associated with it.
The 'Mercedes me Adapter' will connect to your car only when the driver's mobile phone is in the car and the engine is running, so no communication with the phone is available when the driver is away from the car.
(The 'find my car' feature will show the location where the car was last parked, but this is not necessarily the car's current location - e.g., if another driver moved it since - it's not a realtime tracker).
When in the car, communication will work, though keep in mind that it is one way, i.e. the 'Mercedes me Adapter' will display information on your phone, but you cannot send any commands to the car.
It keeps a log of your journeys and refueling stops, which is handy. It also has a 'cockpit' view which I find useful.
It will store the data on your phone, so you can (for example) check the fuel level, or the mileage, or when the next service is due, at any time and from the comfort of your home, i.e. there's no need to be physically in the car when viewing stored data.
With regards fault codes... the 'Mercedes me Adapter' is not a fault code reader. It will not alert the driver if faults existed and will obviously not read the actual codes. What it will do, is let you know when the EML is on, but that's about it (and you'll see the EML on the dash anyway).
With regards the negatives.... it obviously sends information to MB, and yes you will get a call from MB when your next service is due asking if you would like to arrange a booking.
Obviously no one actually reads the pages upon pages of MB's privacy policy that we agree to, so heaven knows what data they are collecting. Personally, I think that if you use a mobile phone, have a PC at home, and use bank cards (not to mention Gmail, Alexa, Netflix, Sky, Amazon, Facebook, Twitter, etc etc) you have long given-up on anonymity and the 'Mercedes me Adapter' will make little difference... but others may disagree.
Other features include calling Mobilo, and SOS calls, though I never had the need to try these.
I do like it and I do use it. I call it 'a useless gimmick that no Mercedes enthusiast can do without'.